Wires and other conductive structures excited by alternating currents and radio fields generally will re-radiate electromagnetic fields that include an electric component and a magnetic component. The electric fields are detectable at great distances, and the magnetic fields are not. The magnetic fields provide a finer source location detail that is very helpful in some applications. The electric fields need to be screened-out to read those finer details.
If the electric field is not too strong at a reception point, it can be screened out completely by a shield screen cage or shield. A ferrite loop antenna inside that cage will only see, and can be used to measure, the magnetic field.
At short ranges, e.g., less than several hundred feet, the concentric lines of force in the magnetic field emitted by wires and other conductive structures can be measured and compared between two magnetic dipole antennas with a short separation distance between them.
Conventional electromagnetic gradiometers (EMG) have used a left hand wound (LHW) and a right hand wound (RHW) horizontal magnetic dipole (HMD) antenna at each end of a twenty foot pole to automatically cancel the electric field and measure the magnetic gradients. The HMD antennas are matched, coaxial, and connected together such that the plane waves they receive from far away will cancel out one another. Any differences in measurements the combination produces will be solely due to the magnetic field differences caused by a local scattering object.
If a radiating wire or conductor is buried underground, two magnetic dipole antennas of an EMG searching above on the ground surface will produce measurements characteristic of their relative positions and orientations. The changes observable over small distances can be dramatic.
Whenever an EMG is relatively close to the radiating wire or conductor, the concentric magnetic lines of force will curve tight enough to be measurable in comparison. Even at ten feet of separation of the antennas. As the EMG receiver moves closer to the target, the magnetic curvatures will get still tighter due to their concentric spreading. The best EMG measurements will therefore be made when the axis of the EMG is orthogonal to the general run of the wire or other conductor underneath.
In practice, the construction and use of an EMG receiver is not so simple. Empirical studies have been needed to find the best ways to excite the underground wires and other conductive structures, what frequencies to use, what power levels, what stand-off distances are needed between transmitters and receivers, what antenna types are best in various situations, and how all these should be varied when the sols comprise clay, silts, sands, cobbles, or other combinations, and when the depths of interest range from a few feet to several hundred feet.